The present invention relates to a process for producing 2-hydroxy-4-methylthiobutanoic acid, which is useful as a feed additive and for other uses. In more particular, it relates to a process for producing 2-hydroxy-4-methylthiobutanoic acid which permits a reduction of the amount of waste water produced and makes the use of organic solvents unnecessary by recycling and reusing the aqueous solution containing ammonium bisulfate and ammonium sulfate formed after the reaction of the process.
A generally known process for producing 2-hydroxy-4-methylthiobutanoic acid comprises subjecting 2-hydroxy-4-methylthiobutyronitrile to hydration and successive hydrolysis by using sulfuric acid. A typical method used for separating the intended 2-hydroxy-4-methylthiobutanoic acid from the reaction liquid resulting from the above-mentioned reaction includes a method comprising the use of an organic solvent, typically represented by methyl isobutyl ketone, as an extraction solvent (see JP-B-7-97970).
However, the method which uses an organic solvent cannot be regarded as the best one, because it requires the cost for solvent and increased investment for equipment necessary for recovering the solvent, increases load on environment, and moreover much care must be taken to avoid the contamination of the final product by the solvent.
Another proposed method is a method comprising neutralizing the ammonium bisulfate contained in the reaction liquid with ammonia to form ammonium sulfate and then separating 2-hydroxy-4-methylthiobutanoic acid from the resulting reaction system by salting-out (see U.S. Pat. No. 4,912,257). This method, however, is not so advantageous, because it consumes large ammounts of sulfuric acid and ammonia.
To say the least, the above-mentioned methods are not preferable from the viewpoint of environmental friendliness, because they use a large amount of sulfuric acid and produce a large amount of ammonium sulfate as a by-product.
On the other hand, a method has been proposed which comprises separating ammonium sulfate from the aqueous layer obtained by layer separation after the reaction thereby to recover an aqueous ammonium bisulfate solution, and recycling the aqueous solution to the reaction system (see Japanese Patent Application No. 9-248592). This method is preferable in point of permitting reduction of the amount of sulfuric acid used, reduction of the amount of by-produced ammonium sulfate and reduction of waste water load. However, it is difficult to reduce environmental load markedly by this method, because it uses an alcohol, such as methanol, for separating ammonium sulfate and ammonium bisulfate from each other.
The present inventors have made extensive study with the object of providing a process for producing 2-hydroxy-4-methylthiobutanoic acid which does not use organic solvents, such as methanol and methyl isobutyl ketone, and moreover permits reduction of the amount of ammonium sulfate by-produced and of waste water load, which is the advantage of the technique disclosed in Japanese Patent Application No. 9-248592. As the result, it has been found that, in the process for producing 2-hydroxy-4-methylthiobutanoic acid by using sulfuric acid, (1) addition of a mixed aqueous solution of ammonium bisulfate and ammonium sulfate to the hydrolysis reaction system promotes the hydrolysis, (2) a layer separation phenomenon takes place rapidly after the above-mentioned reaction, (3) the oil layer obtained by the layer separation can provide, through such operations as neutralization and filtration, a 2-hydroxy-4-methylthiobutanoic acid product equivalent in quality to prior products, and (4) the aqueous layer obtained by the layer separation can be recycled, through succeeding such operations as crystallization and filtration, as a mixed aqueous solution used in (1) above. The present invention has been accomplished on the basis of the above findings.
Thus, the present invention provides a process for producing 2-hydroxy-4-methylthiobutanoic acid comprising the steps of:
(A) contacting an aqueous solution of 2-hydroxy-4-methylthiobutyronitrile with sulfuric acid to obtain an aqueous solution containing 2-hydroxy-4-methylthiobutanoamide,
(B) adding an aqueous solution containing ammonium bisulfate and ammonium sulfate to the solution obtained by step (A), to obtain an aqueous solution containing 2-hydroxy-4-methylthiobutanoic acid,
(C) allowing the aqueous solution obtained by step (B) to separate into two layers of an oil layer and an aqueous layer, and then separating the oil layer and the aqueous layer from each other,
(D) adding ammonia to the oil layer separated in step (C) to neutralize at least part of the ammonium bisulfate in the oil layer to form crystals of ammonium sulfate or crystals of sulfates including ammonium sulfate and ammonium bisulfate, and thereafter removing the crystals from the neutralized oil layer to obtain 2-hydroxy-4-methylthiobutanoic acid,
(E) cooling and/or concentrating the aqueous layer separated in step (C) to obtain an aqueous solution containing ammonium bisulfate and ammonium sulfate as well as crystals of sulfates including ammonium sulfate and ammonium bisulfate, and
(F) recycling all or part of the aqueous solution containing ammonium bisulfate and ammonium sulfate obtained by step (E), to step (B) as the aqueous solution containing ammonium bisulfate and ammonium sulfate.